Adult hippocampal neurogenesis is a dynamic process in which new neurons are continuously generated in the adult brain and integrated into the dentate gyrus, a region that is critical for learning, memory and mood regulation. Dysregulation of this process has been implicated in various psychiatric and neurological disorders, including major depression and epilepsy. Characterizing how these adult- born neurons develop and acquire signaling properties that can affect the local circuitry is important to understand the role of this phenomenon in brain function and pathology. Much of what is known about the electrophysiological properties of these newborn dentate granule cells as they develop has been derived from ex vivo preparations of hippocampal slices. These studies revealed a critical period of plasticity when the cells are around 4 to 6 weeks of age in which they exhibit enhanced synaptic plasticity. This striking observation suggests that there may be a unique, developmentally-regulated role of adult born neurons during a specific time window of maturation. Consequently, a prevalent hypothesis is that newborn granule cells of a particular age exhibit signature patterns of activity in response to environmental stimuli. A direct test of this hypothesis through extracellular single unit recordings has not been possible, however, due to technical limitations that prohibited determining the age of the recorded cell in vivo. To overcome this obstacle, this project is designed to produce a highly specific genetic mouse model (Aim 1) that is amenable to optogenetically-guided tetrode recordings in the dentate gyrus to birthdate, identify and record from single adult-born neurons in freely moving animals (Aim 2). Developing and validating an inducible genetic strategy to target highly proliferative neural progenitors within a narrow time window (i.e. 2 - 4 days) will generate a model in which cohorts of newborn cells can be identified and manipulated with unprecedented precision. This will be a novel resource for the neurogenesis research community to investigate the endogenous function of this population and how its dysregulation may contribute to neural pathology. For the current project, this model will be used to express light-activated opsin channels (channelrhodopsin) in newborn neurons to allow for stimulation and recording of light-responsive putative adult-born granule cells in vivo, via an implanted optical fiber. Completion of these experiments will result in the first description of the firing properties of adult-born granue cells in vivo and the first direct evaluation of whether the critical period of plasticity observedin slice recordings translates to changes in behaviorally relevant neural activity. This innovative approach to address one of the most critical outstanding questions in the field will provide a new genetic model, technique, and dataset to facilitate investigations into the function and dysfunction of adult neurogenesis.